This invention relates to the field of treatment of fungal infections.
The need for novel antifungal treatments is significant, and is especially critical in the medical field. Immunocompromised patients provide perhaps the greatest challenge to modern health care delivery. During the last three decades there has been a dramatic increase in the frequency of fungal infections in these patients (Herbrecht, Eur. J. Haematol., 56:12, 1996; Cox et al., Curr. Opin. Infect. Dis., 6:422, 1993; Fox, ASM News, 59:515, 1993). Deep-seated mycoses are increasingly observed in patients undergoing organ transplants and in patients receiving aggressive cancer chemotherapy (Alexander et al., Drugs, 54:657, 1997). The most common pathogens associated with invasive fungal infections are the opportunistic yeast, Candida albicans, and the filamentous fungus, Aspergillus fumigatus (Bow, Br. J. Haematol., 101:1, 1998; Wamock, J. Antimicrob. Chemother., 41:95, 1998). There are an estimated 200,000 patients per year who acquire nosocomial fungal infections (Beck-Sague et al., J. Infect. Dis., 167:1247, 1993). Also adding to the increase in the numbers of fungal infections is the emergence of Acquired Immunodeficiency Syndrome (AIDS) where virtually all patients become affected with some form of mycoses during the course of the disease (Alexander et al., Drugs, 54:657, 1997; Hood et al., J. Antimicrob. Chemother., 37:71, 1996). The most common organisms encountered in these patients are Cryptococcus neoformans, Pneumocystis carinii, and C. albicans (HIV/AIDS Surveillance Report, 1996, 7(2), Year-End Edition; Polis, M. A. et al., AIDS: Biology, Diagnosis, Treatment and Prevention, fourth edition, 1997). New opportunistic fungal pathogens such as Penicillium marneffei, C. krusei, C. glabrata, Histoplasma capsulatum, and Coccidioides immitis are being reported with regularity in immunocompromised patients throughout the world.
The development of antifungal treatment regimens has been a continuing challenge. Currently available drugs for the treatment of fungal infections include amphotericin B, a macrolide polyene that interacts with fungal membrane sterols, flucytosine, a fluoropyrimidine that interferes with fungal protein and DNA biosynthesis, and a variety of azoles (e.g., ketoconazole, itraconazole, and fluconazole) that inhibit fungal membrane-sterol biosynthesis (Alexander et al., Drugs, 54:657, 1997). Even though amphotericin B has a broad range of activity and is viewed as the “gold standard” of antifungal therapy, its use is limited due to infusion-related reactions and nephrotoxicity (Wamock, J. Antimicrob. Chemother., 41:95, 1998). Flucytosine usage is also limited due to the development of resistant microbes and its narrow spectrum of activity. The widespread use of azoles is causing the emergence of clinically-resistant strains of Candida spp. Due to the problems associated with the current treatments, there is an ongoing search for new treatments.
When the echinocandin caspofungin was approved for sale in 2001, it represented the first new class of antifungal agents to be approved in over a decade. Since that time, two other echinocandin antifungals, anidulafungin and micafungin, have been approved in various markets. Each agent in this class of compound acts by inhibition of β-1,3-glucan synthase, which is a key enzyme in the synthesis of glucan in the cell wall of many fungi. All three of these drugs are made semisynthetically, starting with natural products obtained through fermentation.
The echinocandins are a broad group of antifungal agents that typically are comprised of a cyclic hexapeptide and lipophilic tail, the latter of which is attached to the hexapeptide core through an amide linkage. Although many echinocandins are natural products, the clinically relevant members of this class have all been semisynthetic derivatives. Although the naturally occurring echinocandins possess some degree of anti-fungal activity, they have not been suitable as therapeutics, primarily because of poor aqueous solubility, insufficient potency, and/or hemolytic action. The approved echinocandins are the products of intense efforts to generate derivatives or analogs that maintain or improve upon the glucan synthase inhibition, but do not cause the hemolytic effects. As therapeutic agents, they are attractive compounds in terms of their systemic half-lives, large therapeutic windows, safety profiles, and relative lack of interactions with other drugs. Unfortunately, the poor intestinal absorption of these compounds has relegated them to delivery by intravenous infusion. Although patients receiving these drugs are often hospitalized with serious infections, the ability to transition patients from intravenous delivery in a hospital setting to oral delivery in a home setting would be very desirable, especially considering the course of the regimen commonly exceeds 14 days. In addition, an oral echinocandin may expand the use of this drug class to include patients that present with mild fungal infections.